In a typical wireless network utilizing a coordination function for coordinating transmissions among wireless stations, such a function may be implemented in one of the wireless stations or a coordinator device such as an access point (AP). The wireless stations may communicate via directional transmissions using sector antennas and beamforming antenna arrays. The coordinator device may use omnidirectional transmissions for broadcasts to all wireless stations in all directions (e.g., 360 degrees range). Alternatively, the coordinator may use quasi-omnidirectional transmissions for broadcasts to a wide range, but not necessarily in all directions. In many wireless area networks (WLANs) such as those according to IEEE 802.11 standards, a coordinator station in infrastructure mode is used for providing contention-free access to a wireless communication medium to support Quality of Service (QoS) for certain applications.
In the absence of a coordinator, to provide contention-free channel time reservation, existing techniques use announcement or information exchange among wireless stations in a network to negotiate/reserve the use of the communication medium. For example, IEEE 802.11e Enhanced Distributed Channel Access (EDCA) provides QoS support for certain applications using announcement or information exchange. EDCA defines four Access Categories (ACs) and introduces service differentiation such that certain data traffic uses higher priority parameters to contend for the communication medium.
Further, a frame structure is used for data transmission between wireless stations such as a transmitter station and a receiver station. In one example, a frame structure in a Media Access Control (MAC) layer and a physical (PHY) layer is utilized, wherein in a transmitter station, a MAC layer receives a MAC Service Data Unit (MSDU) and attaches a MAC header thereto, in order to construct a MAC Protocol Data Unit (MPDU). The MAC header includes information such as a source address (SA) and a destination address (DA). The MPDU is a part of a PHY Service Data Unit (PSDU) and is transferred to a PHY layer in the transmitter to attach a PHY header (i.e., PHY preamble) thereto to construct a PHY Protocol Data Unit (PPDU). The PHY header includes parameters for determining a transmission scheme including a coding/modulation scheme. The PHY layer includes transmission hardware for transmitting data bits over a wireless link. Before transmission of a frame from the transmitter station to the receiver station, a preamble is attached to the PPDU, wherein the preamble can include channel estimation and synchronization information.
EDCA allows contention for transmission opportunities (TXOPs), wherein a TXOP is a time interval when a quality of service (QoS) wireless station (STA) may initiate exclusive frame transfer on the wireless medium (e.g., wireless channel). The TXOP may be assigned to the wireless station by a coordinator, or the wireless station may obtain the TXOP by successfully contending for the wireless channel.